Pressure Info

Please note: The following information has been sent to me and gathered from different sources. There is some conflicting information on Diaphone types as well as PSI and CFM numbers. In these cases, this info should serve only as a reference, and not taken as the gospel so to speak. We would hate to hear of any damage caused to these great horns, and we in no way accept any responsibility for any damages that do occur.

According to Gamewell literature, a Type B Diaphone operates on 35 PSI and 1.2 cubic feet per second of free air.

Mike Maderia, President of Maderia Warning Systems, Garwood NJ says…

I run ours at about 48 psi. I want to make sure that I hear it when I am sleeping and I can.”

Adam Smith notes “Variance in sound is a factor of the horn’s condition (how well maintained) and the supply of air. At different feed pressures, a Gamewell horn will have different pitch and timbre. At the pressure where the piston’s motor frequency and the horn’s natural frequency are the same, the horn is in tune and has a smooth mellow note like an air horn. It higher pressures, they have a harsh, more siren-like timbre. With too little air pressure, the piston frequency drops below the horn’s natural resonance, and it becomes really flabby and inefficient, sounding like a fart (sorry!). Different sounds come about as the period of the piston goes in and out of phase with the natural resonant frequency of the horn projector. The resonating column of air in the projector exerts a force on the piston that definitely coerces it towards the horn’s natural frequency. But too much air pressure, and the natural period of the piston’s motor section becomes dominant, at which point the piston alone is setting the pitch, and the horn is not operating in a resonant condition anymore. As with any other kind of air horn or whistle, this is called “overblowing”. Some stations have more than one horn, which is especially common here in Massachusetts, where 2-4 horns per station is typical. In this case, the horns are never quite in tune with each other, and there is always a strong ‘beat frequency’ that pulses at a rate which is the difference(s) in pitch between the two (or more) horns. For example, if one horn is barking at 400 Hz, and another is 405Hz, there will be a strong and audible 5Hz throbbing.”

Adam also makes a note about compressors for operating Diaphones. He says “A piston compressor with a huge air reservoir is what was chosen for use with Gamewell diaphone horns, despite huge tank size and limited reserves of air. Certainly these days, a screw compressor would be the obvious choice for driving Gamewell diaphones, providing 35-40 psi steadily at the required flow rate of 75 CFM.”

Jim Armstrong has 2 complete Diaphones up and running that he obtained several years ago. Jim recently wrote this to me….

“I came across two horns about a year and a half ago; they had been purchased at an auction in a locked box, sort of blind. Several people had fooled around trying to get some noise out of them, with no success. It took a month or so of looking for sounding specs before I got an old sheet from the Gamewell. The problem, of course, was the pressure and the volume of air needed; it didn’t help that both pistons were seized up and I didn’t know if they were supposed to spin or reciprocate. Finally, I used the captive air tank from a water system and a 1″ ball valve; after many tries, I just about knocked my shoes off. Of course the time it decided to sound was the first time I tried without hearing protection. I now have both horns working loudly and whenever I want, with a good-sized tank (tiny, by Gamewell standards) and a good air regulator; they are about 10 feet over my garage on a 1″ galvy pipe.”

“They work best if they work regularly; periods of inactivity will result in sour notes when re-honked. The nasty “blat” is most often a symptom of over-pressure; the later horns came with a tag that says “45 PSI MAX), but I have found that 35 works a whole lot better and has to be easier on the piston (try finding one of THOSE puppies). When I was blowing two horns on one pipe (i.e. all inputs the same for both horns) there was almost no “beat” or overtone. I’m always surprised that even 25 PSI toots sound about the same, so I’m not sure about the pitch/pressure relationship. There were both brass and aluminum pistons made and they definitely produce different sounds, with the heavier brass being deeper. And any numbers other than A22908? Finally, the exquisite interior machining of the brass throat also varies from early to later horns, changing from slots to holes. And if you ever have trouble with your first blast, try placing the piston at its rear-most position. I usually rotate and move it in and out to make sure it’s free.”

SAFETY FIRST! From Adam Smith

Many folks have repurposed LP tanks for air receivers. But, any mechanical engineer will tell you not to, and for a number of reasons.

The other reasons LPG tanks are not advised are primarily design issues, they are designed to be discharged very slowly and their design does not account for metal fatigue from rapid depressurization. Air tanks are designed to withstand repeated rapid evacuation. This has come up many, many times in rec.crafts.metalworking. Also, the drain is only half the rust issue–propane tanks are not built with corrosion allowance accounted for in the wall thickness. Even if you drain the tank after each use, there is still moisture coating the walls. As the walls rust, they present even more surface area for moisture to cling to. Propane and LP gas do not have moisture in them, compressed air always does.

Next, what temperature is that 250PSI rating specified at? Compressed air can reach 300-350F very easily, and ASME specifies that air receiver Max allowable pressure ratings are taken at 475F. Be aware that your LP tank with a 250 PSI rating may give you some false security in that seemingly high number if it’s only counting ambient earth temperatures (120 degrees or so).

When Horn & Whistle did their compressed air safety article, the consensus of the experts was pretty straight forward: use an ASME code air receiver for air, the extra $200 or so spent for a solid rated air tank is worth it where your life is at stake.

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